Application of microwave energy to process various kinds of materials in an efficient, economic, and effective manner, is emerging as an innovative technology and attracting worldwide attention in academia and industries. Microwave heating of materials is fundamentally different from conventional radiation-conduction-convection heating. In the microwave process, the heat is generated internally within the material instead of originating from external heating sources. Microwave heating is a sensitive function of the material being processed.
Microwaves are electromagnetic radiation with wavelengths ranging from 1 mm to 1 m in free space and frequency between approximately 300 GHz to 300 MHz, respectively. Today microwaves at the 2.45 GHz frequency are used almost universally for industrial and scientific applications.
The microwave sintering of ceramic materials has been investigated for over fifteen years and has many advantages over the conventional methods. Some of these advantages include: time and energy saving, very rapid heating rates (&gt;400.degree. C./min), considerably reduced processing time and temperature, better microstructures and hence improved mechanical properties, environment friendly, etc. The use of microwave processing typically reduces sintering time by a factor of 10 or more. This minimizes grain growth. The fine initial microstructure can be retained without using grain growth inhibitors and hence achieve high mechanical strength. The heating rates for a typical microwave process are high and the overall cycle times are reduced by similar amounts as with the process sintering time, for example from hours/days to minutes. And most importantly, the process is a simple, single step process not involving complex steps of hot isostatic pressing (HIP) or hot pressing. All these possibilities have the potential of greatly improving mechanical properties and the overall performance of the microwave processed components with an auxiliary benefit of low energy usage and cost.
The basic powder metallurgy process is a two step process involving the compaction of a metal powder into the desired shape followed by sintering. Typically metal powders in the range of 1 to 120 micrometers are employed. The powder is placed in a mold and compacted by applying pressure to the mold. The powder compact is porous. Its density depends upon the compaction pressure and the resistance of the particles to deformation.
In the sintering process the powder metal compact is heated to promote bonding of the powder particles. The major purpose of the sintering is to develop strength in the compact. The sintering temperature is such as to cause atomic diffusion and neck formation between the powder particles. The basic process is used in industry for a diversity of products and applications, ranging from catalysts, welding electrodes, explosives and heavy machinery and automotive components.
The most important metal powders in use are: iron and steel, copper, aluminum, nickel, Mo, W, WC, Sn and alloys. The traditional powder metallurgy process is neither energy nor labor intensive, it conserves material and produces high quality components with reproducible properties. However, the challenging demands for new and improved processes and materials of high integrity for advanced engineering applications require innovation and newer technologies. Finer microstructures and near theoretical densities in special components are still elusive and challenging.
While ceramics and certain polymers and elastomers absorb microwave energy partly at low temperatures and increasingly at higher temperatures, by and large it is a universal generalization that good conductors such as metals reflect radiation in this wavelength range and hence cannot absorb energy and be heated by microwaves.
This generalization is borne out by the simple fact that in spite of thousands of studies of microwave heating of food, rubber, polymers, ceramics, etc., no one has ever reported an ordinary commercial powder metal part being sintered by microwave energy. Convincing evidence for this is found in the latest textbook on powder metallurgy (Randall M. German, Sintering Theory and Practice, John Wiley, New York, N.Y., 1996), which makes no reference to anyone using microwaves for this task.
The literature reveals the following:
In a paper by Walkiewicz et al. (J. W. Walkiewicz, G. Kazonich, and S. L. McGill, "Microwave heating characteristics of selected minerals and compounds", Min. Metall. Processing (February 1988) pp. 39-42), the authors simply exposed 25 g of some 50 powders of reagent grade chemicals, and some 20 natural minerals to a 2.4 GHz field and reported the temperature attained in the crucible in about 10 minutes or less. Among these samples were powders of some half dozen metals (presumably partly oxidized in the air ambient). These showed modest heating (not sintering) in the range from 120.degree. C. (Mg) to 768.degree. C. (Fe). In the paper by M. Willert-Porada, T. Gerdes, K. Rodiger, and H. Kolaska, entitled "Einsatz von Mikrowellen zum Sintern pulvermetallurgischer Prudukte" (Metall, 50 (11), pp. 744-752 (1996)), the title of which translates to "Utilization of microwaves for sintering of powder-metallurgical products" the only two categories of "powder-metallurgical products" treated are oxides and tungsten carbide-Co composites ("Hartmetallen" in German). In U.S. Pat. No. 4,147,911 issued Apr. 3, 1979, entitled "Method for Sintering Refractories and an Apparatus Therefor", Nishitani describes a method for sintering of refractories using microwaves. He reports that by adding a few percent of electrically conducting powders such as aluminum, the heating rates of the refractories were considerably enhanced. But in this patent there was no mention of the microwave sintering of pure powders of metals. In a paper entitled "Microwave-assisted solid-state reactions involving metal powders" (A. G. Whittaker and D. M. Mingos, J. Chem. Soc. Dalton Trans pp. 2073-2079 (1995)), Whittaker and Mingos reported solid state reaction involving metal powders. They used the high exothermic reaction rates of metal powders with sulfur in microwaves in synthesizing metal sulphides. But no sintering of pure metal or alloy powders is reported in this paper. In a recent textbook on powder metallurgy (Randall M. German, "Sintering Theory and Practice", John Wiley, New York, N.Y. (1996)), the author devotes several pages to microwave heating of oxide and non-oxide ceramics, but makes no reference to anyone using microwave sintering for metals or even suggests that it could work for powdered metals. U.S. Pat. 4,942,278, issued Jul. 17, 1990, entitled "Microwaving of Normally Opaque and Semi-opaque Substances" (Sheinberg et al.) describes the sintering of an oxide-metal composite, basically Cu.sub.2 O and Cu using the absorption by the oxide to cause the temperature to rise into the sintering range. Thus neither theory nor empirical evidence from the literature gives one any hint that one can sinter ordinary typical pressed powder green metal compacts as used by the millions in industry.